Many chemical reactions will proceed at a higher reaction rate or through a different path if one or more of the reactants has been passed through an electrical discharge. The use of an electrical discharge will often allow these reactions to go forward at reasonable speeds at a lower overall temperature than would otherwise be the case. The use of a lower temperature is generally advantageous from a process perspective, since that requires for example less energy.
Roughly speaking one may think of the collisions of reactants with ions and electrons in an electrical discharge as a substitute for the collisions of reactants with other reactants which occur at higher temperatures. Both types of collisions may create activated species which allow the reactions to go forward more rapidly. Naturally, the activated species formed when an electrical discharge is used may not be precisely the same as those formed when the reactants are simply heated to a higher temperature, but often an acceleration of the reaction of interest is nonetheless achieved.
A well-known example of reactions assisted by electrical discharges is the plasma-enhanced chemical vapor deposition of many substances (e.g., silicon or silicon nitride), which is used in the semiconductor industry. In general plasma-assisted reactions have found considerable application in the deposition of thin films.
Plasma-assisted reactions generally take place at low pressures, for example 1 Torr. Low pressures require that the reactions be carried out in pressure vessels which can withstand atmospheric pressure. They also require vacuum pumping systems to keep the pressure low. The pressure vessels may in some cases be limiting as to the size and shape of the materials on which one is operating.
In some prior art references the use of atmospheric pressure plasmas has been disclosed. For example, U.S. Pat. No. 5,399,832 to Tanisaki et al. discloses an apparatus and process to coat or treat surfaces of particles in a glow discharge plasma (strictly speaking, a barrier glow discharge plasma) produced by two electrodes partially coated by a dielectric. The reactor works at atmospheric pressure and employs frequencies of 20 kHz or higher and voltages of 1 to 10 kV.
Plasma torches such as those used for coating or incinerators are examples of atmospheric or above atmospheric plasma reactors. An inert gas is jetted (at supersonic velocities in the case of coatings) between two closely spaced water cooled electrodes. They are kept at a potential difference 50 to 70 V and a current of several hundred amperes passes through the arc discharge plasma so generated, which projects along the path of the gas flow. Very high temperatures (up to 10,000° C.) are achieved, enough to melt powders or to decompose molecules passing through the plasma plume. The residence time can be as short as microseconds so that the completion or extent of reaction may not be complete.
There is a need in the art for more economical and convenient ways to gain the benefits of discharge-assisted reactions.